A leading Iranian journalist and opposition figure has died of a heart attack after spending 10 days on hunger strike in Tehran's notorious Evin prison.

Hoda Saber, a 52-year-old political activist from the opposition Nationalist-Religious movement, was taken to the Modarres hospital in the city after a cardiac complication, which his wife claims was brought on by his hunger strike.

The news of his death coincides with the second anniversary of Iran's disputed presidential election, which gave Mahmoud Ahmadinejad a second term.

Speaking from Tehran, Saber's wife, Farideh Jamshidi, told the Guardian: "My husband died two days ago, but we were unaware of his death until today when someone in the hospital informed one of our friends."

She said Saber stopped eating food and later stopped drinking water in protest at the death of his fellow dissident Haleh Sahabi, She died of a heart attack during scuffles with security forces at the funeral of her father, Ezatollah Sahabi, the leader of the Nationalist-Religious alliance of politicians, on 1 June.

Jamshidi accused prison officials of negligence toward her husband, saying she received reports that they delayed transferring him to hospital for six hours.

"Doctors told us later that they could have saved his life by taking him to the hospital earlier. We were supposed to visit him in the prison tomorrow and now we have to visit his dead body in the cemetery."

Iran's semi-official Fars news agency denied the allegations and said Saber had received medical care before his death. It accused the opposition of politicising his death.

After disappearing for two weeks in July 2010, Saber's family were informed that he had been picked up by security officials and taken to Evin.

When Haleh Sahabi died at her father's funeral, Saber – who had served several jail terms in the past – and his fellow inmate Amir Khosro Dalirsani said in a joint statement that they would go on hunger strike in protest over her death and over the brutal crackdown against protesters by the regime.

Several human rights organisations have issued statements coinciding with the anniversary of the election and many have expressed concerns for those political prisoners arrested since 2009.

The foreign secretary, William Hague, also issued a statement, saying: "Two years after people took to the streets to demand reform, I want it to be known that our attention has not been diverted and we will continue to call on Iran to implement its international human rights obligations."